Most common techniques for transmission rate measurement of sheets include challenging one side of the material with the permeant (test gas) while the other side is swept with a carrier gas. The material is placed in a diffusion cell, separating it into two chambers. The inner chamber is flushed with nitrogen carrier gas and the outer chamber contains the permeant. Molecules of permeant diffuse through the film to the inside chamber and are carried to the sensor by the carrier gas. The computer monitors the increase in water vapor or gas concentration in the carrier gas and it reports that value as the transmission rate (FIGURES 2a and 2b.) This process is perfectly illustrated in the transmission rate units shown above; amount of permeant per unit area per time. Although the typical test sample size is only 50 cm2 it is normalized to either 1 m2 or 100 in2 for purposes of the units. A variety of permeant-specific sensors are used to analyze the concentration in the carrier gas stream. Different sensors can be used depending on the sensitivity required due to barrier level as well as the accuracy desired.
Transmission rate data from flat samples is extremely useful in initial material evaluations, research and development applications and ranking potential materials as candidates for a given package. However, final package configurations can and should also be analyzed. Testing the transmission rate of packages provides insight into the stresses incurred in the packaging process. Full package testing is typically recommended when developing shelf life predictions.
FIGURE 2a. Schematic of an oxygen transmission rate test.
FIGURE 2b. Schematic of a water vapor transmission rate test.
Package transmission rate tests (FIGURE 3a) are conducted under the same principal as sheet measurements. The test gas is supplied to either the interior or exterior of the package while the carrier gas sweeps the opposite side. Typically in the test gas inside / carrier gas outside scenario, a capture volume is required (FIGURE 3b). Since thickness may vary throughout a package and the actual area may be difficult to calculate, the standard transmission rate units for a package are:
FIGURE 3a. Diagram of an oxygen package transmission rate test.
Transmission rate testing does not provide an instantaneous result. For both sheets and packages, it is a dynamic test. Once the test has been started, there is a time of transient behavior prior to reaching equilibrium (FIGURE 4). This time to equilibrium varies between materials, permeants and temperatures. Excellent barriers can take weeks to reach equilibrium while high transmitters might take only a couple hours.
FIGURE 4. Graph of a transmission rate measurement versus time.